The invention relates to an electric testing device for a vehicle combination instrument in general.
In particular, the invention relates to an electric testing device for vehicle combination instruments which comprises display means consisting of segments, a display control in a microcomputer for activating the segments as a function of measured values, for analog values an analog-digital converter arranged in front of the display control, and possibly an input network arranged in front of said converter.
In the prior-art testing of analog-display pointer instruments for vehicles, and particularly automotive vehicles, the pointer instrument is acted on by a given signal corresponding to a simulated measured value and it is determined whether a corresponding display value is obtained.
Vehicle combination instruments with display means consisting of segments formed in particular by liquid crystals cannot be readily tested in this way. In this case, the switch points of the segments must be tested at given measurement values. If the measurement values are represented by resistances of a resistance transmitter for the measurement of the level of the tank, the temperature or the pressure, the testing must traditionally be effected with highly precise resistance decades which are connected to the inputs of the vehicle combination instrument instead of at the measurement transmitters. A testing device having such highly precise resistance decades, which furthermore must be resistant to aging and dimensioned for relatively high loss power, is, however, expensive. Aside from this, the final testing of the vehicle combination instruments which is to be carried out with such a testing device after the instruments have been manufactured is cumbersome and time-consuming, due to the setting of the decades and to the fact that, as a result of the damping provided on many indications of the vehicle combination instrument, it is necessary to wait a considerable amount of time after each change in the adjustment before the display in question can be evaluated.
Furthermore, this resistance-decade testing device is not suitable for reliably detecting other defects in a vehicle combination instrument having display means consisting of segments, in particular to detect shunts between the segments, their control lines or driver connections. A segment will be activated by such a shunt even if, in a given configuration, the display is to remain inactive by the display control.
The object of the present invention, therefore, is to create an electric testing device for vehicle combination instruments which has the features indicated in the second introductory paragraph, is inexpensive and permits a rapid and dependable testing of input circuits with input networks and analog-digital converters as well as testing for undesired shunts of the individual segments.